How long is too long?

I wrote my first novel in about 15 months. It wasn’t very good. I rewrote it several times, and despite all that CPR, it still wasn’t very good. But it was finished. When I got my first book contract, I was obligated to produce a book a year. I finished those first few books in about nine months each.I’m starting a new series, and the time I’ve spent on this book has me thinking I’ve gone back